1. Field of the Invention
The present invention relates to an image processing device and an image processing method.
2. Description of Related Art
A tandem color printer which forms binary images executes image-modification processing of modifying an image for correcting errors in register of colors, errors in register of marks or difference between images on the both sides of a sheet which is caused by shrinkage of a sheet. The nearest-neighbor method is one method used in the processing, but it often causes conspicuous deterioration of picture quality. Therefore, a method of partially using a halftone and pixel-dispersion processing are proposed.
For example, as a means for correcting positioning errors less than one-pixel width, the bilinear method which adjusts an exposure amount of each pixel has been used (see Japanese Patent Publication No. 2007-316154).
Also, an image forming device for correcting errors in register of colors by inserting or deleting pixels to modify an image is proposed (see Japanese Patent Publication No. 2006-270149). This device determines pixel-operation-positions to or from which pixels are inserted or deleted on the basis of predetermined arrangement-parameters.
These methods for correcting a variety of errors in register for a printer can be classified into magnification-correction and deformation-correction.
The magnification-correction is processing wherein pixels are inserted or deleted in a direction of enlarging or reducing an image. Positions where pixels are inserted or deleted vary with lines along the direction of enlarging or reducing the image. Thereby portions where pixels are inserted or deleted are not conspicuous.
The deformation-correction is processing of correcting deformation such as a skew or a bow which is caused by inclination; bend, etc. of a print head. This processing eliminates the deformation of an image. A skew is deformation which is caused when a horizontal or a vertical line is inclined. A bow is deformation which is caused when a straight line is bent.
However, in the magnification-correction and the deformation-correction, a step in an image is caused at a border along which pixels are shifted. Therefore, when the magnification-correction and the deformation-correction are executed individually and successively on an image, deterioration of picture quality is added up and may become too conspicuous to be ignored although deterioration of picture quality caused in each image-modification processing is relatively small. For example, after several kinds of image-modification processing are executed, roughness having two-pixel steps can be caused partially on edges.
Since images are two-dimensional, actual image-modification processing is executed two-dimensionally. When the magnification-correction and the deformation-correction are executed, it is effective to execute processing respectively in a main scan direction and in a sub scan direction for simplifying circuit configuration. Also in this case, a positional relation of defects in an image caused in the first image-modification processing can be changed in the next image-modification processing in a direction perpendicular to the direction in the first image-modification processing, which causes significant deterioration of picture quality.